Seamless steel pipe for steam injection and method for manufacturing the same

ABSTRACT

There is provided a steel pipe for steam injection having high yield stress even at 350° C. The seamless steel pipe for steam injection according to the present invention has a chemical composition comprising, by mass percent, C: 0.03 to 0.08%, Si: 0.05 to 0.5%, Mn: 1.5 to 3.0%, Mo: more than 0.4 to 1.2%, Al: 0.005 to 0.100%, Ca: 0.001 to 0.005%, N: 0.002 to 0.015%, P: at most 0.03%, S: at most 0.01%, and Cu: at most 1.5%, the balance being Fe and impurities. The seamless steel pipe is manufactured by being water cooled after hot working and by being quenched and tempered.

TECHNICAL FIELD

The present invention relates to a seamless steel pipe and a method for manufacturing the seamless steel pipe and, more particularly, to a seamless steel pipe for steam injection and a method for manufacturing the seamless steel pipe for steam injection.

BACKGROUND ART

The steam injection process is used to obtain asphalt from oil sand. In the steam injection process, asphalt is obtained by injecting high-temperature, high-pressure steam into underground oil sand layers.

Steel pipes used in the steam injection process lead steam to oil sand layers. The temperature of the steam is 300 to 350° C. Also, the steam has high pressures. For this reason, steel pipes for steam injection capable of withstanding high temperatures and high pressures are required. More specifically, steel pipes for steam injection having high strength in the temperature range of 300 to 350° C. are required.

-   JP56-29637A (Patent Document 1), JP2-50917A (Patent Document 2) and     JP2000-290728A (Patent Document 3) disclose steel pipes for steam     injection.

DISCLOSURE OF THE INVENTION

The yield strength at 350° C. of all of the steels for steam injection disclosed in these Patent Documents 1 to 3 is lower than steel of X80 Grade of the API5 L standard. More specifically, yield stresses at 350° C. of the steels of these Patent Documents are less than 555 MPa.

It is desirable to use steam of higher temperatures and pressures than ever before in order to obtain more asphalt from oil sand. Steel pipes for steam injection are required to provide greater high-temperature strength than ever before so that high-temperature, high-pressure steam can be used.

It is an object of the present invention is to provide a steel pipe for steam injection having high yield stresses even at 350° C.

The seamless steel pipe for steam injection according to an embodiment of the present invention has a chemical composition comprising, by mass percent, C: 0.03 to 0.08%, Si: 0.05 to 0.5%, Mn: 1.5 to 3.0%, Mo: more than 0.4 to 1.2%, Al: 0.005 to 0.100%, Ca: 0.001 to 0.005%, N: 0.002 to 0.015%, P: at most 0.03%, S: at most 0.01%, and Cu: at most 1.5%, the balance being Fe and impurities. The seamless steel pipe for steam injection is manufactured by being water cooled after hot working and by being quenched and tempered.

Preferably, the chemical composition of the above-described seamless steel pipe comprises, in place of part of Fe, one or more types selected from the group consisting of Cr: at most 1.0%, Nb: at most 0.1%, Ti: at most 0.1%, Ni: at most 1.0%, and V: at most 0.2%.

Preferably, the above-described seamless steel pipe has yield stress of at least 600 MPa at 350° C.

The method for manufacturing a seamless steel pipe for steam injection according to an embodiment of the present invention includes the steps of: heating a round billet having a chemical composition comprising, by mass percent, C: 0.03 to 0.08%, Si: 0.05 to 0.5%, Mn: 1.5 to 3.0%, Mo: more than 0.4 to 1.2%, Al: 0.005 to 0.100%, Ca: 0.001 to 0.005%, N: 0.002 to 0.015%, P: at most 0.03%, S: at most 0.01%, and Cu: at most 1.5%, the balance being Fe and impurities; producing a hollow shell by piercing the heated round billet; producing a seamless steel pipe by rolling the hollow shell; water cooling the seamless steel pipe after rolling; quenching the water cooled seamless steel pipe; and tempering the quenched seamless steel pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram showing the arrangement of manufacturing equipment of a seamless steel pipe for steam injection according to an embodiment;

FIG. 2 is a flowchart showing the manufacturing process of a seamless steel pipe for steam injection according to the embodiment;

FIG. 3 is a schematic diagram showing the temperatures of a billet, a hollow shell and a seamless steel pipe in each step of FIG. 2;

FIG. 4 is a diagram showing the relationship between the tensile test temperature and yield stress of a seamless steel pipe of steel No. 1 in the Example;

FIG. 5 is a diagram showing the relationship between the tensile test temperature and yield stress of a seamless steel pipe of steel No. 2 in the Example; and

FIG. 6 is a diagram showing the relationship between the tensile test temperature and yield stress of a seamless steel pipe of steel No. 3 in the Example.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In the drawings, same or corresponding parts are denoted by the same reference characters and their description will not be repeated.

The present inventors completed the seamless steel pipe for steam injection according to the embodiment of the present invention based on the following findings.

(1) If much molybdenum (Mo) is contained, the yield strength at high temperatures increases. Mo dissolves in steel in a solid solution state and increases the yield stresses of steel at high temperatures. Also, Mo combines with C to form fine carbides and enhances the yield stresses of steel at high temperatures.

(2) If much Mo is contained, weldability decreases. However, weldability is increased by acceleratedly cooling a seamless steel pipe manufactured by hot working and further subjecting the pipe to quenching and tempering. The crystal grains of the steel pipe subjected to accelerated cooling, quenching and tempering are refined. For this reason, the toughness of a weld-heat affected zone and a base metal increases and a decrease in weldability is suppressed.

Hereinafter, embodiments of the seamless steel pipe for steam injection will be described in detail.

Chemical Composition

The seamless steel pipe for steam injection according to the embodiment of the present invention has the following chemical composition. Hereunder, “%” relating to an element refers to a mass percent.

C: 0.03 to 0.08%

Carbon (C) increases the strength of steel. However, if C is contained excessively, toughness decreases and weldability decreases. Therefore, the C content is 0.03 to 0.08%. A preferable lower limit to the C content is 0.04%. A preferable upper limit to the C content is 0.06%.

Si: 0.05 to 0.5%

Silicon (Si) deoxidizes steel. However, if Si is contained excessively, the toughness of steel decreases. In particular, the toughness of a weld-heat affected zone decreases and weldability decreases. Therefore, the Si content is 0.05 to 0.5%. A preferable upper limit to the Si content is 0.3%, and a more preferable upper limit is 0.15%.

Mn: 1.5 to 3.0%

Manganese (Mn) enhances the hardenability of steel and increases the strength of steel. Furthermore, Mn increases the toughness of steel. However, if Mn is contained excessively, the HTC (hydrogen-induced cracking) resistance decreases. Therefore, the Mn content is 1.5 to 3.0%. A preferable lower limit to the Mn content is 1.8%, a more preferable lower limit is 2.0%, and a still more preferable lower limit is 2.1%.

Mo: More than 0.4% to 1.2%

Molybdenum (Mo) increases the high-temperature strength of steel. Specifically, Mo dissolves in steel in a solid solution state and increases the hardenability of steel. The high-temperature strength of steel is increased by an increase in the hardenability. Furthermore, Mo forms fine carbides and increases the high-temperature strength of steel. Furthermore, Mo dissolves in steel in a solid solution state and enhances temper softening resistance. However, if Mo is contained excessively, weldability decreases. More specifically, the toughness of a weld-heat affected zone decreases. Therefore, the Mo content is higher than 0.4% and is at most 1.2%. A preferable lower limit to the Mo content is 0.5%, and a more preferable lower limit is 0.6%.

Al: 0.005 to 0.100%

Aluminum (Al) deoxidizes steel. However, if Al is contained excessively, Al generates cluster-like inclusions and lowers the toughness of steel. Furthermore, if Al is contained excessively, surface defects are apt to occur when a beveled surface is formed on a pipe end. Therefore, the Al content is 0.005 to 0.100%. A preferable upper limit to the Al content is 0.050%, and a more preferable upper limit is 0.030%. A preferable lower limit to the Al content is 0.010%. The Al content in the present invention means the content of acid-soluble Al (what is called Sol. Al).

Ca: 0.001 to 0.005%

Calcium (Ca) combines with S to form CaS. S is fixed by the generation of CaS. Therefore, the toughness and corrosion resistance of steel are increased. Furthermore, calcium restrains the nozzle of a continuous casting apparatus from being clogged during casting. On the other hand, if Ca is contained excessively, Ca is apt to generate cluster-like inclusions and the HIC resistance decreases. Therefore, the Ca content is 0.001 to 0.005%.

N: 0.002 to 0.015%

Nitrogen (N) enhances the hardenability of steel and increases the strength of steel. On the other hand, if N is contained excessively, the toughness of steel decreases. Therefore, the N content is 0.002 to 0.015%.

P: at Most 0.03%

Phosphorous (P) is an impurity. P lowers the toughness of steel. Therefore, the lower the P content, the more preferable. The P content is at most 0.03%.

S: at Most 0.01%

Sulfur (S) is an impurity. S lowers the toughness of steel. Therefore, the lower the S content, the more preferable. The S content is at most 0.01%.

Cu: at Most 1.5%

Copper (Cu) increases the HIC resistance. Specifically, Cu restrains hydrogen from entering steel and restrains the occurrence and propagation of HIC. The above-described effect is obtained if Cu is contained even a little. The Cu content is preferably at least 0.02%. On the other hand, if Cu is contained excessively, the above-described effect becomes saturated. Therefore, the Cu content is at most 1.5%.

The balance of the chemical composition of the seamless steel pipe according to the embodiment is Fe and impurities.

The seamless steel pipe according to the embodiment may also contain, in place of part of Fe, one or more types selected from the group consisting of Cr, Nb, Ti, Ni, and V. These elements increase the strength of steel.

Cr: at Most 1.0%

Chromium (Cr) is an optional element. Cr enhances the hardenability of steel and increases the strength of steel. The above-described effect is obtained if Cr is contained even a little. The Cr content is preferably at least 0.02%, more preferably at least 0.1%, and still more preferably at least 0.2%. On the other hand, if Cr is contained excessively, the toughness of steel decreases. Therefore, the Cr content is at most 1.0%.

Nb: at Most 0.1%

Niobium (Nb) is an optional element. Nb forms carbonitrides and refines the crystal grains of steel. Therefore, Nb increases the strength and toughness of steel. The above-described effect is obtained if Nb is contained even a little. The Nb content is preferably at least 0.003%. On the other hand, if Nb is contained excessively, the above-described effect becomes saturated. Therefore, the Nb content is at most 0.1%.

Ti: at Most 0.1%

Titanium (Ti) is an optional element. Ti suppresses the occurrence of surface defects of cast pieces during continuous casting. Furthermore, Ti forms carbonitrides and refines the crystal grains of steel. Therefore, Ti increases the strength and toughness of steel. The above-described effect is obtained if Ti is contained even a little. The Ti content is preferably at least 0.003%. On the other hand, if Ti is contained excessively, the above-described effect becomes saturated. Therefore, the Ti content is at most 0.1%.

Ni: at Most 1.0%

Nickel (Ni) is an optional element. Ni enhances the hardenability of steel and increases the strength and toughness of steel. The above-described effect is obtained if Ni is contained even a little. The Ni content is preferably at least 0.02%. On the other hand, if Ni is contained excessively, the above-described effect becomes saturated. Therefore, the Ni content is at most 1.0%.

V: at Most 0.2%

Vanadium (V) is an optional element. V forms carbonitrides and refines the crystal grains of steel. Therefore, V increases the strength and toughness of steel. The above-described effect is obtained if V is contained even a little. The V content is preferably at least 0.003%. On the other hand, if V is contained excessively, the toughness of steel decreases. Therefore, the V content is at most 0.2%.

Manufacturing Method

The seamless steel pipe in accordance with this embodiment is acceleratedly cooled after hot working. The seamless steel pipe is further quenched and tempered after accelerated cooling. The yield stress of the seamless steel pipe manufactured by the above-described process at 350° C. is at least 600 MPa. In addition, the seamless steel pipe has high toughness because the seamless steel pipe has a micro-structure in which the crystal grains are refined. Therefore, a decrease in the weldability of steel is suppressed in spite of the high Mo content. Hereinafter, a method for manufacturing the seamless steel pipe according to this embodiment will be described in detail.

Manufacturing Equipment

FIG. 1 is a block diagram showing one example of a manufacturing line for a seamless steel pipe for steam injection according to this embodiment. Referring to FIG. 1, the manufacturing line includes a heating furnace 1, a piercer 2, an elongation rolling mill 3, a sizing mill 4, a holding furnace 5, a water cooling apparatus 6, a quenching apparatus 7, and a tempering apparatus 8. Between the apparatuses, a plurality of transfer rollers 10 are arranged. In FIG. 1, the quenching apparatus 7 and the tempering apparatus 8 are included in the manufacturing line. However, the quenching apparatus 7 and the tempering apparatus 8 may be arranged separately from the manufacturing line. In other words, the quenching apparatus 7 and the tempering apparatus 8 may be arranged off-line.

Flow of Manufacturing

FIG. 2 is a flowchart showing the manufacturing process of the seamless steel pipe according to this embodiment. FIG. 3 is a diagram showing a change in the surface temperature of a material being rolled (a round billet, a hollow shell, and a seamless steel pipe) with respect to time during the manufacture.

Referring to FIGS. 2 and 3, in the method for manufacturing the seamless steel pipe for steam injection according to this embodiment, first, a round billet is heated by the heating furnace 1 (S1). Successively, the heated round billet is hot worked into a seamless steel pipe (S2 and S3). Specifically, the round billet is piercing-rolled into a hollow shell by the piercer 2 (S2), and further, the hollow shell is rolled into the seamless steel pipe by the elongation rolling mill 3 and the sizing mill 4 (S3). The seamless steel pipe produced by hot working is heated to a predetermined temperature as necessary by the holding furnace 5 (S4). Successively, the seamless steel pipe is water cooled (acceleratedly cooled) by the water cooling apparatus 6 (S5). The water cooled seamless steel pipe is quenched by the quenching apparatus 7 (S6), and is tempered by the tempering apparatus 8 (S7). Hereunder, each of the steps is explained in detail.

Heating Step (S1)

First, a round billet is heated by the heating furnace 1. The heating temperature is preferably 1050 to 1300° C. Heating the round billet at a temperature in this temperature range provides high hot workability of the round billet at the piercing-rolling time, and surface defects are suppressed. Also, heating the round billet at a temperature in this temperature range restrains crystal grains from coarsening. The heating furnace is a well-known walking beam furnace or rotary furnace, for example.

Piercing-Rolling Step (S2)

The round billet is taken out of the heating furnace 1, and the heated round billet is piercing-rolled to produce a hollow shell by the piercer 2. The piercer 2 has a well-known configuration. Specifically, the piercer 2 includes a pair of conical rolls and a plug. The plug is arranged between the conical rolls. The piercer 2 is preferably a toe angle piercer. This is because piercing-rolling can be performed at a high pipe expansion rate.

Rolling Step (S3)

Next, the hollow shell is rolled. Specifically, the hollow shell is elongated and rolled by the elongation rolling mill 3. The elongation rolling mill 3 includes a plurality of roll stands arranged in series. The elongation rolling mill 3 is a mandrel mill, for example. Successively, the elongated and rolled hollow shell is sized by the sizing mill 4 to produce a seamless steel pipe. The sizing mill 4 includes a plurality of roll stands arranged in series. The sizing mill 4 is a sizer or a stretch reducer, for example.

The surface temperature of the hollow shell rolled by the rearmost roll stand of the plurality of roll stands of the sizing mill 4 is defined as a “finishing temperature”. The finishing temperature is measured, for example, by a temperature sensor disposed on the outlet side of the rearmost roll stand of the sizing mill 4. The finishing temperature is preferably at least the A₃ point (more specifically, the A_(c3) point) as shown in FIG. 3. The finishing temperature is more preferably at least 900° C., and still more preferably at least 950° C. The A_(c3) point of a seamless steel pipe having the chemical composition of the present invention is 750 to 950° C. At a finishing temperature of 900° C. or higher, in a hollow shell being subjected to sizing, the heat loss caused by roll heat dissipation is small. Therefore, the temperature unevenness of the produced seamless steel pipe can be reduced.

Reheating Step (S4)

A reheating step (S4) is carried out as necessary. In other words, the reheating step need not necessarily be carried out. In the case where the reheating step is not carried out, in FIG. 2, the process proceeds from step S3 to step S5. Also, in the case where the reheating step is not carried out, in FIG. 1, the holding furnace 5 may not be provided.

In the case where the reheating step is carried out, the produced seamless steel pipe is charged into the holding furnace 5 and is heated. Thereby, the temperature unevenness of the produced seamless steel pipe is reduced. The heating temperature in the holding furnace 5 is the A_(r3) point to 1100° C. If the heating temperature is lower than the A_(r3) point, the α phase precipitates and the micro-structure becomes nonuniform, so that the variations in strength increase. On the other hand, if the heating temperature exceeds 1100° C., the crystal grains coarsen. The heating time is preferably 1 to 30 minutes.

Water Cooling Step (S5)

The seamless steel pipe produced in step S3 or the seamless steel pipe reheated in step S4 is water cooled (acceleratedly cooled) by the water cooling apparatus 6. The surface temperature of the seamless steel pipe just before water cooling is substantially the same as the finishing temperature or the heating temperature in the holding furnace. That is, the surface temperature of the seamless steel pipe just before water cooling is at least the A₃ point, preferably at least 900° C., and still more preferably at least 950° C.

The water cooling apparatus 6 includes a plurality of rotating rollers, a laminar stream device, and a jet stream device. The plurality of rotating rollers are arranged in two rows, and the seamless steel pipe is arranged between the plurality of rotating rollers arranged in two rows. At this time, the rotating rollers arranged in two rows come into contact with a lower portion on the outer surface of the seamless steel pipe. When the rotating rollers rotate, the seamless steel pipe rotates around the axis thereof. The laminar stream device is arranged above the rotating rollers, and sprinkles water onto the seamless steel pipe from above. At this time, the water sprinkled onto the seamless steel pipe forms a laminar stream. The jet stream device is disposed near the end of the seamless steel pipe placed on the rotating rollers, and injects a jet stream from the end of the seamless steel pipe into the steel pipe. By use of the laminar stream device and the jet stream device, the outer and inner surfaces of the seamless steel pipe are cooled at the same time.

Preferably, the water cooling apparatus 6 cools the seamless steel pipe until the surface temperature of the seamless steel pipe reaches a temperature of at most 450° C. In other words, the water cooling stop temperature is at most 450° C. With the water cooling stop temperature at most 450° C., the crystal grains of the seamless steel pipe are refined further by quenching in the subsequent step. As a result, the toughness of the seamless steel pipe is improved further.

The cooling rate of the water cooling apparatus 6 is preferably at least 10° C./sec. The water cooling apparatus 6 may be an apparatus other than the above-described apparatus including the rotating rollers, the laminar stream device, and the jet stream device. For example, the water cooling apparatus 6 may be a water tank. In this case, the seamless steel pipe produced in step S3 is immersed in the water tank, and is cooled. Such a cooling method is called “dip cooling”. Also, the water cooling apparatus 6 may consist of the laminar stream device only. In sum, the type of the water cooling apparatus 6 is not subject to any restriction.

Quenching Step (S6)

The seamless steel pipe water cooled by the water cooling apparatus 6 is quenched. The quenching temperature, is preferably higher than the A_(c3) point and at most 1000° C. When the seamless steel pipe is heated to the above-described quenching temperature, the micro-structure of the seamless steel pipe transforms from bainite to a fine austenitic structure. That is, reverse transformation takes place. At this time, the crystal grains are refined. That is, by performing accelerated cooling in step S5, the refining of crystal grains can be promoted in the quenching step.

If the quenching temperature is lower than the A_(c3) transformation point, the reverse transformation does not take place sufficiently. On the other hand, if the quenching temperature exceeds 1000° C., the crystal grains coarsen. The soaking time in quenching is preferably 10 seconds to 30 minutes. After soaking at the quenching temperature, the seamless steel pipe is water cooled.

Tempering Step (S7)

The quenched steel pipe is tempered. The tempering temperature is at most the A_(c1) point, and is regulated based on desired dynamic properties. By performing tempering, the yield stress of the seamless steel pipe of the present invention at 350° C. can be regulated to at least 600 MPa. The variations in the tempering temperature are preferably ±10° C., and more preferably ±5° C. If the variations in the tempering temperature are small, the desired dynamic properties are achieved easily.

In the above-described manufacturing method, accelerated cooling is performed (S5) and thereafter quenching is performed (S6). By use of these steps, the refining of crystal grains is promoted. For this reason, the produced seamless steel pipe has excellent toughness. Therefore, although the seamless steel pipe in accordance with this embodiment contains much Mo, a decrease in toughness is restrained and also a decrease in weldability is restrained.

Furthermore, by quenching and tempering the seamless steel pipe having the above-described chemical composition, the yield stress of the seamless steel pipe at 350° C. can be regulated to at least 600 MPa.

Example

A plurality of seamless steel pipes for steam injection having various chemical compositions were manufactured, and yield stresses at normal temperature (23° C.) to 360° C. were examined.

Examination Method

A plurality of billets having the chemical compositions given in Table 1 were manufactured.

TABLE 1 Steel Chemical composition (mass %, balance Fe and impurities) No. C Si Mn P S Cu Cr Ni Mo Ti V Nb Al Ca N Inventive 1 0.06 0.23 2.20 0.009 0.0014 0.04 0.32 0.03 0.76 0.003 0.005 <0.001 0.042 0.0016 0.0030 example Inventive 2 0.06 0.24 2.13 0.008 0.0010 0.04 0.30 0.39 0.74 <0.001 0.005 <0.001 0.025 0.0021 0.0027 example Comparative 3 0.06 0.33 1.49 0.007 0.0014 0.27 0.27 0.13 0.11 0.009 0.05 0.001 0.035 0.0015 0.0027 example

Referring to Table 1, the chemical compositions of billets of steel No. 1 (inventive example) and steel No. 2 (inventive example) were within the range of the chemical composition of the present invention. On the other hand, the chemical composition of steel No. 3 (comparative example) was out of the range of the chemical composition of the present invention. Specifically, the Mn content of steel No. 3 was less than the lower limit to the Mn content of the present invention. Furthermore, the Mo content of steel No. 3 was less than the lower limit to the Mo content of the present invention. The contents of elements of steel No. 3 other than Mn and Mo were within the range of the chemical composition of the present invention. All of the N contents of steel No. 1 to 3 were within the range of 0.002 to 0.015%. Incidentally, the Ti content of steel No. 2 and the Nb contents of steel No. 1 and No. 2 were at the level of impurities.

Each of the produced billets was heated by the heating furnace. Successively, the billets were piercing-rolled by the piercer to produce hollow shells. Successively, the hollow shells were elongated and rolled by the mandrel mill, and were then sized by the sizer, whereby a plurality of seamless steel pipes were produced. Successively, the seamless steel pipes of steel No. 1 and No. 2 were water cooled (acceleratedly cooled). The finishing temperature of all of the seamless steel pipes was 1100° C., and the water cooling stop temperature was 450° C. On the other hand, for the seamless steel pipe of steel No. 3, air cooling was performed after rolling.

Each of the seamless steel pipes after cooling was quenched. In all of the seamless steel pipes, the quenching temperature was 950° C. and soaking was performed for 40 minutes. After quenching, the seamless steel pipes were tempered. The tempering temperature was 650° C. and soaking was performed for 30 minutes. By use of the above-described steps, seamless steel pipes for steam injection were manufactured.

Yield Stress

From a central portion of the wall thickness of each of the manufactured seamless steel pipes, a plurality of tensile test specimens conforming to ASTM A370 were sampled. And by using the tensile test specimens, the tensile test conforming to ASTM E21 was conducted in the temperature range of room temperature (23° C.) to 360° C. More specifically, in each test No., the tensile test was conducted by using two tensile test specimens at the temperatures of 23° C., 100° C., 200° C., 300° C., 350° C. (only steel No. 3), and 360° C. (only steel No. 1 and No. 2). The yield stress and tensile strength were determined on the basis of the test results. In this embodiment, the yield stress was determined by the 0.5% total elongation method.

Examination Results

Table 2 shows the yield stress and tensile strength of the seamless steel pipes of each steel No. FIG. 4 shows the relationship between the tensile test temperature and yield stress and tensile strength of the seamless steel pipe of steel No. 1. FIG. 5 shows the relationship between the tensile test temperature and yield stress and tensile strength of the seamless steel pipe of steel No. 2. FIG. 6 shows the relationship between the tensile test temperature and yield stress and tensile strength of the seamless steel pipe of steel No. 3. The symbol ♦ in FIGS. 4 to 6 indicates yield stress. The symbol ▪ indicates tensile strength.

TABLE 2 Tensile test temperature (° C.) Steel No. Strength 23 100 200 300 350 360 1 Yield stress (MPa) 720/721 708/696 671/671 653/659 — 625/618 Tensile strength (MPa) 785/786 765/755 749/747 761/757 — 732/732 2 Yield stress (MPa) 748/748 718/717 681/683 667/669 — 639/645 Tensile strength (MPa) 810/810 778/778 758/758 779/777 — 753/761 3 Yield stress (MPa) 630/628 582/594 581/582 580/574 561/557 — Tensile strength (MPa) 698/700 652/664 657/658 666/660 665/657 —

The “yield stress” columns in Table 2 show the yield stress (MPa) of corresponding steel Nos. at each temperature. Two values are shown as the yield stress at each temperature. For example, “720/721” is entered in the yield stress column of steel No. 1 at 23° C. In this case, “720/721” indicates that the tensile stresses obtained from two tensile test specimens were 720 MPa and 721 MPa. Similarly, the “tensile strength” columns in Table 2 show the tensile strength (MPa) of corresponding steel Nos. at each temperature.

Referring to Table 2 and FIGS. 4 to 6, in all of the temperature ranges, the yield stresses of the seamless steel pipes of steel No. 1 and steel No. 2 were larger than the yield stresses of the seamless steel pipe of steel No. 3. Furthermore, the yield stresses of steel No. 1 and steel No. 2 at 350° C. were at least 600 MPa. On the other hand, the yield stresses of steel No. 3 at 350° C. were less than 600 MPa.

The above is a description of an embodiment of the present invention, and the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be modified as appropriate without departing from the spirit of the present invention. 

1. A seamless steel pipe for steam injection having a chemical composition comprising, by mass percent, C: 0.03 to 0.08%, Si: 0.05 to 0.5%, Mn: 1.5 to 3.0%, Mo: more than 0.4 to 1.2%, Al: 0.005 to 0.100%, Ca: 0.001 to 0.005%, N: 0.002 to 0.015%, P: at most 0.03%, S: at most 0.01%, and Cu: at most 1.5%, the balance being Fe and impurities, wherein the seamless steel pipe is manufactured by being water cooled after hot working and further by being quenched and tempered.
 2. The seamless steel pipe according to claim 1, wherein the chemical composition comprises, in place of part of Fe, one or more types selected from the group consisting of Cr: at most 1.0%, Nb: at most 0.1%, Ti: at most 0.1%, Ni: at most 1.0%, and V: at most 0.2%.
 3. The seamless steel pipe according to claim 1, wherein the seamless steel pipe has yield stress of at least 600 MPa at 350° C.
 4. The seamless steel pipe according to claim 2, wherein the seamless steel pipe has yield stresses of at least 600 MPa at 350° C.
 5. A method for manufacturing a seamless steel pipe for steam injection, comprising the steps of: heating a round billet having a chemical composition comprising, by mass percent, C: 0.03 to 0.08%, Si: 0.05 to 0.5%, Mn: 1.5 to 3.0%, Mo: more than 0.4 to 1.2%, Al: 0.005 to 0.100%, Ca: 0.001 to 0.005%, N: 0.002 to 0.015%, P: at most 0.03%, S: at most 0.01%, and Cu: at most 1.5%, the balance being Fe and impurities; piercing the heated round billet to produce a hollow shell; rolling the hollow shell to produce a seamless steel pipe; water cooling the seamless steel pipe after rolling; quenching the water cooled seamless steel pipe; and tempering the quenched seamless steel pipe.
 6. The method of manufacturing a seamless steel pipe according to claim 5, wherein the chemical composition of the round billet comprises, in place of part of Fe, at least one types selected from the group consisting of Cr: at most 1.0%, Nb: at most 0.1%, Ti: at most 0.1%, Ni: at most 1.0%, and V: at most 0.2%. 